Topic: 1977 CB750 Jetting carb after modifications

[eurban]

are you running a stock airbox?  I'm not sure dropping your slows is going to help since you're showing extremely lean in low RPM.  What are your fuel screws set to?  I was also having starting problems on my K7 but after resetting my valve clearances and adjusting the choke cable, starts right up now, at least it's a place to start.

Dropping the slows?  Stock is #35 which is what he ordered.

[wsup55]

are you running a stock airbox?  I'm not sure dropping your slows is going to help since you're showing extremely lean in low RPM.  What are your fuel screws set to?  I was also having starting problems on my K7 but after resetting my valve clearances and adjusting the choke cable, starts right up now, at least it's a place to start.

Dropping the slows?  Stock is #35 which is what he ordered.

for some reason, i was thinking they were 40s

[Dawks]

Thanks for the help fellas, Have changed back to the #115 Mains , dropped the needle height and adjusted the pilot screws to 3 turns. also changed the fuel to 98 octane and give it another run Still lean in the low revs but flatterned the Air/fuel ratio  (14 - 15  2000-4000) out (between 12 to 13.5 right thru to 8500) Final power and torque remained about the same but beefed up both from 4000 through to 7000
I suspect from what I've read thru this post if I increase the slows it will sort the lower range. Ping basically disappeared after winding out the pilot screws and changing fuel. checked the fuel pump and found the outlet of above was plugged and looks like it was done in factory (a bit weird but that's how it is) decided to leave for now whilst sorting the other stuff.
Thanks for the heads up on the jet size. I've now realised I should have ordered #38 or #42 Slows as my standards are #35 so I'll try again. saved me waiting the 2 weeks delivery to find this out.       

[eurban]

Why would you order the 38s?  Sort out your accelerator pump malfunction before you worry about your idle jet size.  It must function for these carbs to work correctly . . . The stock size pilots should be large enough as long as they are actually clean.  The RPM ranges for your a/f mix doesn't tell you very much.  Rather you should be taking note of what the mix is at various throttle positions.  Mark your throttle with some tape and note the a/f just off idle, 1/4,1/2,3/4 and WOT. 

[rkubin]

I agree with eurban- while I did not install 836 kit, I did install K&N pods and a yoshi style (mostly) open header, which greatly increases breathing.  The accel pump is critical to proper function.  Before I fixed that, I also had the IMS screws out 3-4 turns just so it would run.  With pump fixed, they are now back to 1.5-2 turns.  I am running 125 mains, which seem to be about right, though I may try dropping one position to address a slight flat spot around 5000 rpm.  Still breaking in new rings, so haven't thrashed it too hard yet.  Will update once I have a few more miles on it.

One trick on the accel jets- the passage for cyl #4 was still blocked after 1,2,3 were squirting.  I plugged 1,2,3 by putting a piece of rubber tubing over the nozzle, then sprayed carb cleaner through again, which finally opened up the #4 path way.

[eurban]

checked the fuel pump and found the outlet of above was plugged and looks like it was done in factory (a bit weird but that's how it is) decided to leave for now whilst sorting the other stuff.      

Please elaborate or show some pics of what you are talking about.

[Dawks]

Thanks for the feedback
I'm tempted to drill the outlet for the fuel pump, I guess if it gives me problems can simply re-plug. I've pulled off no.2 bowl again and checked it, it defiantly been left blanked off when manufactured all other parts in place so should work. It was made for the Aussie market so may been some weird emissions thing.
I've recently fitted original 4 into 4 pipes sourced  from CMNS, if these were the original issue I don't imagine fuel would off been an issue as the baffling and silencer is so severe they smothered the motor, it sounded like Nana's scooter, using a dremel with 1" cutting wheel and purpose made slide hammer managed to get rid of the silencers and used an 18"long 1/2 drill to get thru the baffles to let it breath. Still not overly loud but respectable, Still looks original and didn't have to go near the chrome. Thanks again     

[eurban]

Can you take a picture of the blocked off outlet?

[rkubin]

Hi folks- thought I would post an update and ask for some further advice.  After chasing a few other problems - header nuts were loose, timing was off a bit - I am back to the carbs.  It idles nice, but there is a major stumble when I crack the throttle open - it almost slows, then takes off, almost like a major turbo lag...  Once the rpm's get up there, it seems fine again.  I have checked the accel pump and it seems to be working fine still.  Could this be an issue with the needles?  Should I try raising them up one position?  They are currently 2nd position from top and are 125s, again with fairly open yosh style 4-1 and K&N pods.

[TwoTired]

I have checked the accel pump and it seems to be working fine still.  Could this be an issue with the needles? 

There's "working", and there's "working properly".
I had mine "working",  And still had throttle stumble.
The two check valve balls have to seat fully.  Had one leaking.  Cleaned it so they couldn't back flow.  It was better, but still stumbled.
Found out the mickey mouse ears on the pump diaphragm had squeezed nearly shut.  Drilled them out to match the bore size, and wow what a strong shot of fuel went into each carb throat, and the stumble was gone and the bike became well mannered.

I have the stock exhaust and a K&N air filter in the stock box. 77-CB750 F  (Which uses the same style carbs as the K7)

The k7 calls for the slide needle clipped in position 1.  With your set up, I would almost guarantee you have to open that up more but how much will have to be determined by plug chops or a session on the dyno.  The accelerator pump really should take care of all the throttle response issues, provided it is working properly.  IMO.  The steady state mid throttle position mixture is primarily determined by the slide needles.

Cheers,

[rkubin]

well, good advice again TwoTired!  I took the pods off and checked the accel pump jets again- low and behold there was NO gas squirting out!  pulled the carbs again (getting pretty good at it now) and pulled #2 bowl and accel pump.  There was a bunch of black sludge in the bottom of the bowl - as I have 2 filters from the tank (screen on petcock and inline filter) I am thinking that the accel pump passages/check valves were not completely clean and ended up depositing gunk into the bowl.  I cleaned again (had previously also drilled out the mickey mouse ears during the last go round) and checked function on the bench.  Put everything back together, started her up and acceleration is MUCH better!

Regarding the plug chop, this is something I have not done before.  My understanding is that you run at the desired rpm (4000-5000?) under load for some time (2 min?), then pull in the clutch and kill the engine, then check the plugs for color/condition looking for signs of lean or rich mixture.  Lean would then indicate needles should be raised?

[TwoTired]

Glad you got it working.

Throttle position is more important than RPM.   If you are doing the throttle valve, mid position is likely best.  You want new/clean plugs and operation at only at that throttle position to get a relevant deposit reading.

Lower the clip/raise the needle = richer mixture.

Cheers,

[rkubin]

thanks Two - I did a search on plug chop and came across a number of your previous posts.  Sounds like you have an engineering background...  One comment struck home with my experience test riding yesterday - don't expect sport bike response when you crack the throttle.  My last ride was a Yamaha R6 - crack the throttle and it takes off!  I tried varying the rate of throttle opening and found that it was actually pretty strong acceleration if I was a little slower on the twist.  Physics of carburation at work!

Just wanted to thank you for your obvious interest and willingness to put up with noobs.  btw, I am a mech. engineer by training, but spent much of my career in management.  Good to get back to the roots!

[gnarlycharlie4u]

Thanks, it will be another week before I tear into mine but i am suspecting the accel pump.  Will check the floats again while i'm in there.

Also, just in case you need it, check this out.
http://www.hondachopper.com/garage/carb_info/accelpump_valve_repair/page1.html

Here I made it into a printable PDF.

[lucky]

Getting ready to get into jet troubles huh. First off, what pods did you get? If they are the cheap emgo ones, you might have no end of trouble.
Also, on the 77, you cant just go up in idle jet sized as they are pressed in. In many cases though, you dont need to.
Have you tried to adjust your low speed mix or just decided that increasing jets is what you need?
the mix screw offer a very good range of adjustment. In is lean, out is richer. Also, have you changed the needle height at all?
I would most likely turn out the mix screw a little and lower the clip to raise the needle. I know some here have done pods and never changed the idle jets.

Also, do not assume your idle jets are clean just cause you saw light and sprayed carb cleaner. you have to make sure EVERY hole in them is clean and the best way is with a strand of copper wire to poke gently through the holes.
Dont just assume that because you made these changes that you need to increase your jet sizes.

Also, have you checked what color your plugs are after you idle for a bit? That can tell you a lot.

The 1978 has only one groove and one clip. You would have to add a shim to raise the needle. A real pain in the a!@#$# with those complicated style slides.
Honda just keeps messing up. The earlier carbs were much easier to work on and the needles had 5 positions. Everything was easier and simpler.

[lucky]

hi there- as it turns out I am in a very similar situation, so will post here rather than start new question.

I have a 1977 K7 with just under 10k miles that I have restored/customized, frame-up over last 1.5 years.  I put on K&N pods and a yoshi style 4-1 (minimal baffle), so went up 2 main jet sizes to 125 (stock is 115) and have them set at second position from top.  Pilot screws were all set at 1.5 turns out per manual (they were originally 3-4 turns out when disassembled).  Carbs were completely disassembled and cleaned.  Stock ignition.

Bike started right up, but will not run/idle without choke on, even when hot.  I am assuming that indicates a lean mixture.  I do not have carb sync gauges, so have not checked sync.  Will check plug condition tomorrow.

Any suggestions on next steps?  Will adjusting the pilot screws help with the mixture?

thanks!

Makes perfect sense. The pilot air screws were out about as far as they will go without falling out and there was a reason. When you turned them back in to the stock setting it leaned the engine out again.
The first 1/4 throttle is controlled by the idle jet and the pilot air screw ONLY.
You are confused about the mainjets. A mainjet is a brass screw with a hole drilled in it. It has no "positions".
The NEEDLE in the slide that goes down through the mainjet has grooves and a clip.
The needle with clip in the second position from the top is about right. MISTAKE I will change that - move the clip down to the second groove from the bottom to richen it. Sorry for the error..
Now all you have to do is richen up that pilot air screw on each carb and you will get some better throttle response. Turn them all out 1/2 turn at a time and check the throttle response. You will notice a  difference.

[thewudd]

for those that have the 77 and have replaced the main jets...where did you get them?  i haven't been able to find the ones i need.  if you might have some of the larger sizes and are willing to part w/ them...let me know.

[lucky]

hi there- as it turns out I am in a very similar situation, so will post here rather than start new question.

I have a 1977 K7 with just under 10k miles that I have restored/customized, frame-up over last 1.5 years.  I put on K&N pods and a yoshi style 4-1 (minimal baffle), so went up 2 main jet sizes to 125 (stock is 115) and have them set at second position from top.  Pilot screws were all set at 1.5 turns out per manual (they were originally 3-4 turns out when disassembled).  Carbs were completely disassembled and cleaned.  Stock ignition.

Bike started right up, but will not run/idle without choke on, even when hot.  I am assuming that indicates a lean mixture.  I do not have carb sync gauges, so have not checked sync.  Will check plug condition tomorrow.

Any suggestions on next steps?  Will adjusting the pilot screws help with the mixture?

thanks!

You can only turn out the mixture screw so far before it just falls out.
Every bike I have ever had that had a free breathing intake required one step larger on the idle jet - every single time.

If putting the choke on helps smooth out the advancement of throttle, it is a solid sign of a lean mixture.

Also you will need more fuel in the midrange. So you need to raise the needle.
IF it has no clip and grooves you will need to put shims under the needle.
Small brass washers. Usually the smallest ones from the hardware store are about .022 thousandths and I have used 2 of those on each carb and that is about equal to on groove position on a needle with clip arrangement. About .050 thousandths.
Do not forget to sand or file the back side of the washers because they have a burr on them from being punched out.

[lucky]

are you running a stock airbox?  I'm not sure dropping your slows is going to help since you're showing extremely lean in low RPM.  What are your fuel screws set to?  I was also having starting problems on my K7 but after resetting my valve clearances and adjusting the choke cable, starts right up now, at least it's a place to start.

Dropping the slows?  Stock is #35 which is what he ordered.

for some reason, i was thinking they were 40s

A #35 slow jet is .013 thousandths.
A #40 slow jet is .015 thousandths.

That is a .002 thousands difference. about the thickness of a human hair.

[lucky]

Getting ready to get into jet troubles huh. First off, what pods did you get? If they are the cheap emgo ones, you might have no end of trouble.
Also, on the 77, you cant just go up in idle jet sized as they are pressed in. In many cases though, you dont need to.
Have you tried to adjust your low speed mix or just decided that increasing jets is what you need?
the mix screw offer a very good range of adjustment. In is lean, out is richer. Also, have you changed the needle height at all?
I would most likely turn out the mix screw a little and lower the clip to raise the needle. I know some here have done pods and never changed the idle jets.

Also, do not assume your idle jets are clean just cause you saw light and sprayed carb cleaner. you have to make sure EVERY hole in them is clean and the best way is with a strand of copper wire to poke gently through the holes.
Dont just assume that because you made these changes that you need to increase your jet sizes.

Also, have you checked what color your plugs are after you idle for a bit? That can tell you a lot.

!977 CB750's do not have needles with clips.CORRECTION They do have adjustable needles.

[lucky]

This thread was posted in 2009.

Did you ever get this bike straightened out?

Was the accelerator pump system and squirt nozzles working?

[lucky]

I have recently been working hard to get the 1977-78 CB750 carbs to work properly with pods or stacks.
I have been working on Honda carbs from age 16 it the first Honda shop I worked at until now. I am 62 now.

I was not successful for the first time to get these 77-78 carbs to work properly.
It is not the jets or the needle shimming problem.

And you are right that with some choke it will run right without the stock filter but if you try to ru pods or stacks forget about it. It will not run right.

The problem is that the stock filter was like a giant choke itself.
The air opening was small and it had a paper filter.
It was like having the choke on.

But you cannot just run it with the choke on because the the idle circuit will not work right. You plugs will get carboned up.

The only way i can see the 77-78 CB750 carb to work right is that the tapered needle on the slide must have the needle changed to a different taper.
Also the air jets may have to be altered and the jets too.
Othewise a complete workover on a flow bench by an expert.

Might as well just buy a set of CR carbs for $800.
Or if you have a chopper get the duel carb Mikuni set up.

If you are really a good mechanic maybe you can just buy 4 Mikuni carbs (old style) and get those jetted and adjusted.

Personally I am going to go back to the ugly big black stock air box and see if I can get some chrome spray for the outside of it. Not kidding.

I have spent a lot of time working on these 77-78 carbs and just jetting and shims on the needle does not get good permanent results.

If anyone knows which tapered needle would fix the problem, I would like the number or letters of that needle.

[Retro Rocket]

If you are really a good mechanic maybe you can just buy 4 Mikuni carbs (old style) and get those jetted and adju

You can buy Mikuni's racked to suit our bikes or you can buy intake rubbers that allow the use of the older VM29's that were on the Kawasaki 900's. You can also buy a freer flowing air filter for the stock airbox and open up the intake hole for more flow.....Tapered needles won't make much difference because it all comes down to the smooth flow of air required for these bikes to run well, still air + velocity stacks = much improved running....

[Godffery]

Has anyone tried replacing the 77-78 (single grove) needles with earlier 69-76 (5 grove) nedles?

[lucky]

so I plan to check the float levels today- what is the correct level?  does this have to be done with the engine running?

thanks

Float levels need to be set with the carbs off of the motorcycle and no gas in them on the bench. A must do. Make sure you have new float needles and seats if they have not been changed for a couple of years.

The accelerator pump system and nozzles MUST be working.
Bench check by filling the float bowls with fuel and actuate the throttle a couple times while looking into the intake and making sure you see gas squirt from the spray nozzle of each carb. USe a bright LED pocket flashlight.


I am going to save you lots of time on the set up.

1977 carbs.
Use the stock air filter housing and filter.******
4 into 1 exhausts or any aftermarket exhausts.
#120 main jet.
#42 idle jet.
Needle in the stock position.
Mixture screws open 3/4 turn.


ALTERNATIVE. (experimental but workable) You will be very close.

1977 carbs.
PODS.
Aftermarket exhaust.
#120 main jet.
#42 idle jet.
Needle clip two notches down from stock.
Mixture screw open 3/4 turn.

There it is. I just saved you lots of work.